This project is designed to provide information about the populations of motor units that make up large limb muscles in mammals. The scope of work includes studies of the electrophysiological and morphological characteristics of spinal cord motoneurons, the organization of synaptic inputs to them, and the relationship of these central nervous system factors to the mechanical, histochemical and anatomic properties of the muscle fibers (termed "muscle units") innervated by the motoneurons. Current work on this project is largely focused on neuroanatomic studies and computer modeling of individual, functionally-identified motoneurons, with emphasis on the fundamental features that control dendritic morphology and the influence of dendritic anatomy on the electrical properties of neurons and mechanisms of information processing in dendrites. We have devised a relatively simple stochastic model that is useful to isolate the key factors that control dendritic morphology. This approach is being used to compare the fundamental dendritic structure of several groups of cat motoneurons, as well as the morphologies of motoneuron dendrites in two groups of motoneurons during postnatal development. We are using the original data and computer- generated dendrites to explore the electrophysiological consequences of different dendritic structures. We are now extending these studies to examine the quantitative characteristics of dendritic trees in three dimensions.